This invention relates to methods of coating optical surfaces and in particular, to a new and useful process for coating an ophthalmic lens so that the finished lens has an anti-static and/or anti-reflective surface.
The build-up of static charge on plastic elements (especially plastic ophthalmic lenses coated with organo-silica based abrasion-resistant coatings, i.e. silica-polyorganosiloxane or polyorganosiloxane) attracts dust and is unacceptable in many applications (e.g., polycarbonate safety lenses in steel mills, cotton mills and coal mines). In the case of eyewear, these dust particles cause light scattering or haze which can severely limit the visual acuity of the wearer and necessitates frequent cleaning.
Certain topical treatments are commercially available for the prevention of static charge build-up, but these topical treatments are short lived and must be continually repeated.
Another way of preventing the build-up of static charge on plastic lenses is to imbibe anti-static agents into the plastic materials. However, these anti-static agents are known eye irritants and may not be suitable for ophthalmic purposes. Furthermore, these anti-static agents are designed to migrate to the surface where they can interfere with the coating/substrate interface.
It is also often desirable in many applications to reduce the reflectance of an optical surface. By reducing the reflectance of light impinging upon the surface of an optical element, a greater percentage of incident light will be transmitted through the optical element.
When optical elements are molded out of a polymeric plastic substrate, the reflectance is commonly reduced by vacuum deposition of single or multiple film layers which are designed and fabricated to reduce reflectance by interference effects. These layers require a high level of skill and complex equipment to manufacture on a large scale. Also, when some of these coatings are exposed to moist or otherwise hostile environments, they will deteriorate rapidly. Specifically, unless care is taken in the design and construction of such coatings, exposure to hostile environments may reduce the adherence of the coating to the substrate and the coating may be peeled or otherwise separated from the optical element.
There has been considerable use in recent years of an electrical discharge in order to form flexible thin films of solid organic material upon the surface of a substrate. Thin electrical discharge may be maintained in the sparking, corona or glow region, of an electrical phenomenon.
A glow discharge may be defined as a silent discharge without sparks and having a space potential gradient in the vicinity of the cathode resulting in a potential difference near the cathode which is considerably higher than the ionization potential of the surrounding gas. Typical structure of the glow discharge is identified by a steep potential gradient at the cathode and operating primarily by electron liberation by positive ion bombardment at the cathode. In relation to a corona discharge, a glow discharge is characterized by a much lower potential or voltage and a higher current than a corona discharge. Unlike a corona discharge which is a reversible discharge situation, the glow discharge occurs after the sparking or breakdown potential is exceeded and is an irreversible change which has occurred in the electrical circuit.
Accordingly, it is a principal object of the present invention to reduce the reflectance of and to prevent the buildup of static charge on the surfaces of an optical element.
It is another object of the invention to reduce the reflectance of and to prevent the build-up of static charge on the surface of plastic ophthalmic lenses coated with abrasion-resistant coatings.
It is a further object of the invention to reduce the reflectance of the surfaces of an optical element without causing any coatings on the optical element to peel or otherwise separate from the optical element.